22. What is the link-state retransmit interval, and what is the command to set it?
OSPF must send acknowledgment of each newly received link-state advertisement (LSA). LSAs are retransmitted until they are acknowledged. The link-state retransmit interval defines the time between retransmissions. We can use the command ip ospf retransmit-interval to set the retransmit interval. The default value is 5 seconds.

23. When routes are redistributed between OSPF processes, are all shortest path first algorithm (SPF) metrics preserved or is the default metric value used?
The SPF metrics are preserved. The redistribution between them is like redistribution between any two IP routing processes.

24. How do I stop individual interfaces from developing adjacencies in an OSPF network?
To stop routers from becoming OSPF neighbors on a particular interface, issue the passive-interface command at the interface.

25. When I have two type 5 link-state advertisements (LSAs) for the same external network in the OSPF database, which path should be installed in the routing table?
When you have two type 5 LSAs for the same external network in the OSPF database, prefer the external LSA that has the shortest path to the Autonomous System Boundary Router (ASBR) and install that into the IP routing table. Use the show ip ospf border-routers command to check the cost to the ASBR.

26. Should I use the same process number while configuring OSPF on multiple routers within the same network?
OSPF, unlike Border Gateway Protocol (BGP) or Enhanced Interior Gateway Routing Protocol (EIGRP) does not check the process number (or autonomous system number) when adjacencies are formed between neighboring routers and routing information is exchanged.

27. Can we have OSPF run over a GRE tunnel?
Yes we can have OSPF run over a GRE tunnel.